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The planet that hums

By Robert Coontz

THEY live underground. They are everywhere but seem to come from nowhere.
They barely exist, but never leave. If sounds have shadows, they are the shadows
of a sound.

Researchers call them the background free oscillations of the Earth. But last
year, when a pair of Japanese geophysicists named Naoki Suda and Kazunari Nawa
dredged them out of a mass of seismic data, some people called them a hum.
That’s a comforting thought: a mystic Om, perhaps, or just the warm, cosy sound
of a planet going about its business.

Don’t try to tune in, you’ll never hear it, though. The Hum is far too low
for human ears to detect and is so feeble that a single 5.5-magnitude earthquake
can blot it out. That’s just as well because, if you could hear it, the Hum
might drive you mad.

“It’s a very messy noise,” says Hiroo Kanamori, a geophysicist at the
California Institute of Technology. Messy because the Hum is not one note but
fifty, crammed into less than two octaves. Their pitches range between 2 and 7
millihertz. Musically speaking, that’s about sixteen octaves below middle C.
Speeded up and amplified so you could hear it, the result would be a
Stockhausenesque cacophony. Imagine sitting down at a piano and slamming down
every note within reach, while somebody next to you does the same thing on a
piano a quarter tone out of tune. “It would be like banging a trash can,”
Kanamori says. Endlessly.

The individual notes are pleasant enough. They are the natural tones that the
Earth makes whenever something—an earthquake, a meteor, …